Currently, nonaqueous electrolyte secondary batteries represented by lithium secondary batteries are widely mounted on portable devices, and so on. For these nonaqueous electrolyte secondary batteries, principally LiCoO2 is used as a positive active material. However, the discharge capacity of LiCoO2 is about 120 to 130 mAh/g.
As a material of a positive active material for a lithium secondary battery, a solid solution of LiCoO2 and other compounds is known. Li[Co1-2xNixMnx]O2 (0<x≦½), a solid solution having an α-NaFeO2-type crystal structure and formed of three components: LiCoO2, LiNiO2 and LiMnO2, was published in 2001. LiNi1/2Mn1/2O2 or LiCo1/3Ni1/3Mn1/3O2 that is one example of the aforementioned solid solution has a discharge capacity of 150 to 180 mAh/g, and is also excellent in terms of charge-discharge cycle performance.
In contrast with so-called a “LiMeO2-type” positive active material as described above, so-called a “lithium-excess-type” positive active material is known in which the composition ratio Li/Me of lithium (Li) to the ratio of a transition metal (Me) is greater than 1, with Li/Me being, for example, 1.25 to 1.6 (see, for example, U.S. Pat. No. 6,677,082, U.S. Pat. No. 7,135,252, Japanese Patent No. 4877660 and JP-A-2010-50079). Such a material can be denoted as Li1+αMe1−αO2 (α>0). Here, β=(1+a)/(1−α) when the composition ratio Li/Me of lithium (Li) to the ratio of a transition metal (Me) is β, and therefore, for example, α=0.2 when Li/Me is 1.5.
U.S. Pat. No. 6,677,082 and U.S. Pat. No. 7,135,252 describe a positive active material for a lithium secondary battery, which has the general formula of xLiMO2.(1−x)Li2M′O3 (0<x<1), The documents also describe that M is at least one or more selected from Mn, Co and Ni and that M′ is Mn. The documents show that the positive active material enriched in Li has a stabilized crystal structure, and a lithium secondary battery having a high discharge capacity is obtained by using this active material.
Japanese Patent No. 4877660 describes the invention of “an active material for a lithium secondary battery comprising a solid solution of a lithium transition metal composite oxide having an α-NaFeO2-type crystal structure, wherein the composition ratio of Li, Co, Ni and Mn contained in the solid solution satisfies Li1+(1/3)xCo1−x−yNi(1/2)yMn(2/3)x+(1/2)y (x+y≦1, 0≦y, 1−x−y=z), (x, y, z) is represented by a value present on the line of or within a heptagon ABCDEFG having point A (0.45, 0.55, 0), point B (0.63, 0.37, 0), point C (0.7, 0.25, 0.05), point D (0.67, 0.18, 0.15), point E (0.75, 0, 0.25), point F (0.55, 0, 0.45) and point G (0.45, 0.2, 0.35) as apexes, in a Li[Li1/3Mn2/3]O2(x)-LiNi1/2Mn1/2O2(y)-LiCoO2(z)-system triangular phase diagram, and the intensity ratio of the diffraction peak of the (003) line and the (104) line in X-ray diffraction measurement is I(003)/I(104)≧1.56 before charge-discharge, and I(003)/I(104)>1 at the end of discharge”, (claim 1), and shows that by using the positive active material enriched in Li, a lithium secondary battery, which has a high discharge capacity, and particularly has a high discharge capacity in a potential range of 4.3 V or less, is obtained.
On the other hand, a lithium secondary battery, wherein a positive active material containing a lithium transition metal composite oxide and a negative active material containing graphite and amorphous carbon (noncrystalline carbon) are combined, is well known (see, for example, JP-A-2010-50079, JP-A-2011-54371, JP-A-2012-15051 and JP-A-2012-84322).
JP-A-2010-50079 describes the invention of “a nonaqueous electrolyte secondary battery comprising: a positive electrode containing a positive active material; a negative electrode containing a negative active material; and a nonaqueous electrolyte having lithium ion conductivity, wherein the positive active material is a lithium-containing transition metal composite oxide having a layered structure and represented by the general formula: Li1+x(NiaMnbCoc)O2+a (x+a+b+c=1, 0.7≦a+b, 0<x≦0.1, 0≦c/(a+b)<0.35, 0.7≦a/b≦2.0, −0.1≦α≦0.1), and the nonaqueous electrolyte contains a lithium salt having an oxalate complex as an anion” (claim 1), and an object of the invention is to provide “a nonaqueous electrolyte secondary battery using as a positive active material a lithium-containing transition metal oxide, which has a layered structure and in which the main component of the transition metal includes two elements: nickel and manganese, wherein the nonaqueous electrolyte secondary battery is excellent in power characteristics and low in cost” (paragraph [0010]).
JP-A-2010-50079 also describes that for the negative active material, “a noncrystalline carbon-coated graphite formed by coating a graphite material with noncrystalline carbon is suitably used from the viewpoint of output/input characteristics” (paragraph [0033]); and a nonaqueous electrolyte secondary battery, wherein a positive active material is “Li1.07Ni0.42Co0.09Mn0.42O2” and a negative active material is “a graphite coated on the surface with noncrystalline carbon”, as Example 8 (paragraphs [0042] and [0056] to [0058]”.
JP-A-2011-54371 describes the invention of “a lithium ion secondary battery comprising: a positive electrode containing a positive active material capable of inserting/extracting lithium ions; a negative electrode containing, as a negative active material capable of inserting/extracting lithium ions, a carbon material in which an noncrystalline carbon material constituting 55% by weight to 85% by weight (inclusive) of the whole negative active material and a graphite material constituting 15% by weight to 45% by weight (inclusive) of the whole negative active material are mixed; and a nonaqueous electrolyte solution for wetting the positive electrode and negative electrode” (claim 1), and the invention “has as an object the provision of a lithium ion secondary battery, of which input/output characteristics can be improved” (paragraph [0007]).
JP-A-2011-54371 also describes that “by including as a negative active material a carbon material in which an noncrystalline carbon material as a principal material and a graphite material as an auxiliary material are mixed, input characteristics can be improved because the noncrystalline carbon material has a high charge capacity and high retention as compared to the graphite material, and power characteristics can be improved because the graphite material can keep the battery voltage high and has a small reduction in power in an end stage of discharge as compared to the noncrystalline carbon material” (paragraph [0009]), and “for the positive active material, a lithium-nickel-manganese-cobalt composite oxide is used; namely, it is a composite oxide represented by the general formula: LixNiyMnzCo(1−y−z−w)AwO2 and having a layered crystal structure; here, in the general formula, x satisfies 0<x<1.2, and y and z satisfy y+z<1” (paragraph [0020]).
JP-A-2012-15051 describes the invention of “a negative electrode for a lithium ion secondary battery, which comprises: a negative current collector; and a negative composite layer disposed on the negative current collector and containing crystalline carbon and noncrystalline carbon as a negative active material, wherein the negative composite layer includes a plurality of layers, a layer closer to the negative current collector has a higher content of crystalline carbon, a layer farther from the negative current collector has a higher content of noncrystalline carbon, and in the layer closest to the negative current collector, the content of crystalline carbon is higher than the content of noncrystalline carbon” (claim 1), “a lithium ion secondary battery comprising: the negative electrode for a lithium ion secondary battery according to any one of claims 1 to 6; and a positive electrode including a positive current collector, and a positive composite layer containing a positive active material” (claim 7), and “the lithium ion secondary battery according to claim 7, wherein the positive active material is a lithium layered composite oxide represented by the general formula: LixNiyMnzCo(1−y−z−w)AwO2 [wherein 1.0≦x≦1.2, y+z+w<1, y≧z, 0≦w≦0.01, and A is at least one selected from the group consisting of Li, Al, Cr, Mg, Ti, B, F and W” (claim 8). JP-A-2012-15051 also describes that “a lithium ion secondary battery using only a graphite carbon material as a negative active material has a problem in terms of cycle life characteristics because input/output characteristics are poor, and therefore degradation of the surface is significant in repetition of high-rate charge-discharge although a high capacity is achieved; further, in . . . , the mixing ratio of graphite is low, and input/output characteristics can be kept high, but it is difficult to achieve further capacity enhancement; therefore, capacity enhancement and improvement of cycle life characteristics of the lithium ion secondary battery are desired” (paragraph [0008]), and “the weight ratio of crystalline carbon and noncrystalline carbon contained in the negative composite layer is not particularly limited, but the amount of crystalline carbon contained in the whole layer is preferably larger than the amount of noncrystalline carbon; specifically, the weight ratio of crystalline carbon and noncrystalline carbon is preferably 1.3:1 to 10:1, especially preferably 1.5:1 to 5:1” (paragraph [0029]).
JP-A-2012-84322 describes a method for production of a lithium ion secondary battery, comprising an assembly step of preparing a battery which contains in a battery case an electrode body having a positive active material and a negative active material, and an electrolyte solution containing a difluorophosphoric acid salt, wherein “the positive active material is LixMO2 (wherein M is Ni, or contains at least any one of Al, Ti, V, Cr, Mn, Fe, Co, Cu, Zn, Mg, Ga, Zr and Si in addition to Ni as a principal component), and satisfies 1.04≧X≦1.15”, and “particles of the negative active material include graphite and noncrystalline carbon, and the ratio of the noncrystalline carbon in the particles of the negative active material ranges from 2.5 to 7.1 wt %” (claim 1).
So-called a “lithium-excess-type” positive active material described above generally has a discharge capacity higher than that of so-called a “LiMeO2-type” positive active material, and thus has such a feature that by going through high-potential charge (high-potential formation) of 4.5 V (vs. Li/Li+) or more in an initial charge-discharge step, a high discharge capacity is achieved even if the charge potential is subsequently decreased.